The present invention relates generally to cathode ray tubes, and more particularly to a cathode ray tube including a saddle type of horizontal coil used in a deflection yoke.
An important aspect of performance for a television monitor is its ability to correctly align the individual color components (red, green and blue). Mis-convergence describes how far apart the three electron beams spread from one another within a pixel. Ideally, the beam hits all three dots in the group without hitting any adjacent groups. Mis-convergence is a quantitative measurement of the lack of convergence of the three electron beams. From a viewer's perspective, in the case of mis-convergence the resulting image will have a shadowed appearance.
A deflection yoke is used to control the convergence of the three electron beams (red, green and blue) in a cathode ray tube (CRT) by changing the winding distribution in the horizontal and vertical coils to compensate for mis-convergence. For example, U.S. Pat. No. 5,838,099 discloses one such deflection yoke, which can be seen in FIG. 2, which depicts a perspective view showing a pair of saddle type horizontal deflection coils.
When correcting mis-convergence, in general there are four parameters that affect convergence as controlled by the horizontal coil of a deflection yoke--PQV, S1, S2 and S3. Of these, PQV is the main control parameter for the deflection yoke. The other parameters S1, S2 and S3 are critical to correct the mis-convergence in the middle screen area.
FIGS. 1a-d depict the plus pattern on the CRT screen, in which FIG. 1a depicts the PQV related mis-convergence; FIG. 1b depicts the S1 related mis-convergence; FIG. 1c depicts the S2 related mis-convergence; and FIG. 1c depicts the S3 related mis-convergence. As evident in FIG. 1a, the PQV related mis-convergence concerns the mis-convergence of the red and blue electron beams at the edges of the screen. In contrast, the S3 related mis-convergence concerns the mis-convergence of the red and blue electron beams at the middle of the horizontal edges. Similarly, the S1 and S2 related mis-convergence concern the edges and middle of the screen, respectively. The relationships between the various parameters are given below: ##EQU1##
Normally, one adjusts the convergence error by using the horizontal coil, which creates a pincushion type magnetic field. Using the horizontal coil, the mis-convergence termed PQV, XH (which is the edge of X-axis horizontal convergence error) and the mis-convergence termed HCR (which is the same location of XH, but a different type of convergence error) are corrected to zero. Of course, S1, S2 and S3 are simultaneously controlled to within desired values by the magnetic field generated by the horizontal coil.
Viewer preferences for larger and flatter television screens requires cathode ray tubes with wider deflection angles. As the screen in a CRT becomes larger and flatter, and as the deflection angle becomes correspondingly wider, it becomes more difficult to adjust the mis-convergence using conventional methods. In particular, in such CRT's it is difficult to correct the mis-convergence in the middle section of the screen.
In an attempt to overcome the problem of correcting the mis-convergence in the middle of the screen, a correction device has been installed on the deflection yoke. However, this complicates the manufacturing process and requires additional components. Moreover, the correction resulting from this device is not entirely sufficient for the larger CRT's.
To correct the convergence error, it is necessary to provide a strong magnetic field (i.e., a strong pincushion-type patterned distorted magnetic field) using the horizontal coil. Basically, the main parameter PQV is relatively easy to correct. However, the parameters S1, S2 and S3 (which are termed middle of the convergence) cannot be adjusted using only the horizontal coil in the conventional manner. Consequently, the mis-convergence related to these parameters remains out of the desired range. To correct this middle convergence error it is necessary to provide a strong magnetic field.
FIGS. 3a-d depict a conventional method for correcting this convergence error, including middle area convergence error. To do so, one places a small magnet on the deflection yoke front portion, which can be seen in FIGS. 3b-d. In practice, however, it is too difficult to correct convergence error that consists of combinations of S1, S2 and S3, e.g., S1 with S2, or S2 with S3, or S1 with S3 using this method. Moreover, this method is also problematic from a standpoint of geometry in that the design of the horizontal coil should be symmetrical to create predictable magnetic fields.
Another conventional technique uses an additional correction device placed on the deflection yoke, as seen in FIG. 4. Unfortunately, this technique increases the cost and decreases the reliability. Furthermore, this design requires specific fine tuning and an engineering design.
FIGS. 5a-c depict one such additional correction device in more detail. FIG. 5a depicts the correction device in top view and FIG. 5b depicts the correction device in side view. The device includes a circuit, as set forth in FIG. 5c. The convergence parameters PQV, S1, S2 and S3 before the addition of the correction device are depicted in FIG. 5d. After addition of the correction device, the improvements in the convergence is shown in FIG. 5e. As apparent in FIG. 5e, the combination of the various mis-convergences is difficult to correct using such a correction device.
The present invention is therefore directed to the problem of developing a method and apparatus for adjusting the mis-convergence in the middle area of a cathode ray tube without requiring the use of a dedicated correction device, which method and apparatus can be employed in larger and flatter CRT screens.